1. Field of the Invention
The present invention relates to the fields of microwave signal sources, solid state high-frequency electronics and microelectronics. More particularly, the present invention relates to a microstrip stabilized quantum well resonance-tunneling generator which generates electromagnetic waves of millimeter and submillimeter wavelength range.
2. Description of the Prior Art
Quantum effects including resonant tunneling effects are widely used in modern solid state and semiconductor electronics for application in the fields of communications, radiovision, introscopy, molecular spectroscopy, Earth atmosphere monitoring, and astrophysics, medicine and biology. The resonant-tunneling coupled quantum well diode structures were found to be the most promising to operate at terahertz frequencies since the tunneling charge transfer processes are extremely fast (see, e.g., U.S. Pat. No. 4,745,452).
Nevertheless, the expansion of operation frequencies of the resonant tunneling diode based generators towards the terahertz range is a problem of considerable complexity. The main reasons are fast increase of the energy losses at higher frequencies, difficulties in fabrication of the small size resonator systems, the increasing role of parasitic inductance and other parameters of the circuitry connecting an active element with an external resonator system.
Microwave generators based on the resonant tunneling quantum well diode structures with millimeter-band oscillation frequencies of 200, 420 and 720 GHz at room temperature were discloses in: E. R. Brown, T. C. L. G. Sollner et al., J. Appl. Phys. 64(3), 1519-1529 (1988), Appl. Phys. Lett. 55(17), 1777-1779 (1989), Appl. Phys. Lett. 58(20), 2291-2293 (1991).
The resonant tunneling diodes consisted of two AlAs barriers separated by a quantum well of GaAs (or InAs). The resonator structures used to obtain the oscillations were rectangular metallic waveguides with dimensions dependent on the oscillation frequency. A highest oscillation frequency of 712 GHz was obtained with a 0.030×0.015 cm2 rectangular waveguide resonator. The DC bias was provided by a coaxial line with a whisker contact, wherein the coaxial line served to stabilize the diode at all frequencies below the band of interest, the whisker contact served to decrease the electric losses in coupling the electrical circuit. The highest power generated was about 0.3 μW.
The oscillation frequency was determined by the equivalent circuit impedance, including the equivalent parameters of the diode itself (the difference conductance, the frequency-dependent series resistance, the diode capacitance and the quantum well inductance) as well as external electric circuit parameters. To improve the high frequency characteristics of the device, the in-plane dimensions of the diode were made less than 2 microns. The technological difficulties of the fabrication of the small size rectangular waveguide resonators as well as fast decrease of its high-frequency characteristics restrict the possibilities of wide application of this type of devices.
Microstrip resonators, processing an intermediate position between confined cavity waveguide resonator systems used in microwave radiophysics and open cavity resonators used in optics, are widely used in high-frequency electronics.
These resonators are made of planar materials that allows low cost manufacturing processes and wide variation of its dimensions. It is important that microstrip lines may be integrated into the hybrid integrated circuits with the use of conventional thin-film technology. The resonator quality factor Q may be made rather high in high-frequency range, in particular, by the use of more complicated construction of the planar resonator. An example of such a high-Q resonator is disclosed in U.S. Pat. No. 5,825,266.
Another way of improving Q is to use high-Tc superconductors as a microstrip material. An example of such a high-Q resonator is disclosed in U.S. Pat. No. 6,021,337. A thickness of a superconducting coating placed on a dielectric layer in such a device exceeds one micron and may be a hundred of microns, thus allowing sufficient penetration of an electromagnetic field into the superconductor. At high frequencies, the application of striplines is limited by parameters of dielectrics, by an increase of the energy losses and an appearance of unwanted modes of oscillation.
“Microstrip Stablized Semiconductor Asymmetrical Quantum Well Structure Generator for Millimeter and Submillimeter Wavelength Range”, A. A. Beloushkin et al., Superlattices and Microstructures, Vol. 22, No. 1, p. 19-23 (1997), discloses a device which combines the advantages of resonant-tunneling diodes and microstrip lines used as a microwave resonator. A resonant-tunneling quantum well structure was theoretically designed on the basis of self-consisted computer simulation and grown by molecular beam epitaxy (MBE) on a semi-insulated wafer of GaAs. The structure consisting of two 4.5 nm AlAs barriers, divided by a single 4.0 nm GaAs quantum well, includes spacer layers preventing impurity segregation out of heavily doped contact layers into an active quantum zone.
The system of coplanar contacts provided a minimum time delay in a negative differential conductivity (NDC) region of a current-voltage (I-V) curve due to a decrease of a capacitance and a series resistance of the device. The contact areas composed of the heavily doped GaAs and of the Cr/Au ohmic contacts were formed by vacuum deposition on the mesas of 0.01-0.025 mm in diameter. The microstrip resonator was designed as quarter wave T-coupled microstrip line with one end short-circuited. The Cu strips were patterned on a 1.5-mm-thick dielectric substrate (Teflon) with double-plane metallization. The line had a width of 2 mm and a length of 70 mm. The resonant tunneling diode was connected to resonator by short metallic conductors configured to provide a minimum level of inductive and resistive parasitic parameters. A room temperature microwave generation was achieved at frequency range 1-10 GHz with double barrier resonant tunneling diode as an active element (microwave power of 0.01-0.1 mW).
The efficiency of the use of a microstrip resonator in a shorter wavelength range is determined in a considerable degree by the quality of the wave impedance matching of the active element (resonant tunneling diode) and resonator. This matching is difficult to achieve when the active element and the microstrip resonator are fabricated separately, as was the case in the disclosed device.